I have a C++ program which reads files from the hard disk and does some processing on the data in the files. I am using standard Win32 APIs to read the files. My problem is that this program is blazingly fast some times and then suddenly slows down to 1/6th of the previous speed. If I read the same files again and again over multiple runs, then normally the first run will be the slowest one. Then it maintains the speed until I read some other set of files. So my obvious guess was to profile the disk access time. I used perfmon utility and measured the IO Read Bytes/sec for my program. And as expected there was a huge difference (~ 5 times) in the number of bytes read. My questions are:
(1). Does OS (Windows in my case) cache the recently read files somewhere so that the subsequent loads are faster?
(2). If I can guarantee that all the files I read reside in the same directory then is there any way I can place them in the hard disk so that my disk access time is faster?
Is there anything I can do for this?
1) Windows does cache recently read files in memory. The book Windows Internals includes an excellent description of how this works. Modern versions of Windows also use a technology called SuperFetch which will try to preemptively fetch disk contents into memory based on usage history and ReadyBoost which can cache to a flash drive, which allows faster random access. All of these will increase the speed with which data is accessed from disk after the initial run.
2) Directory really doesn’t affect layout on disk. Defragmenting your drive will group file data together. Windows Vista on up will automatically defragment your disk. Ideally, you want to do large sequential reads and minimize your writes. Small random accesses and interleaving writes with reads significantly hurts performance. You can use the Windows Performance Toolkit to profile your disk access.